1. Field of the Invention
The invention relates to a power converter; in particular, to a power converter and an operating method thereof capable of automatically extending an on-time to improve its transient response.
2. Description of the Prior Art
In recent years, power supply circuits have been widely used in different electronic products, such as portable electronic products or computer products. The power supply circuits can provide voltage or current conversion or provide power with fixed voltage or current for electronic products to use. In the power supply circuits, a power integrated circuit (Power IC) is one of necessary active components.
Please refer to FIG. 1 and FIG. 2. FIG. 1 illustrates a schematic diagram of a power converter with a conventional constant on-time (COT) system structure. FIG. 2 illustrates a timing diagram of output waveforms of the power converter of FIG. 1. As shown in FIG. 1 and FIG. 2, in the conventional COT system structure, every output duty cycle of the power converter 1 includes a fixed on-time and a variable off-time. Since the off-time cannot be zero, there will be a minimum off-time ΔToff(min) existed in the output duty cycle. When the system loading becomes heavier, a pulse width modulation signal PWM outputted by an on-time calculator 14 has higher frequency to provide a faster transient response.
However, since there is the minimum off-time ΔToff(min) existed in the output duty cycle, the pulse width modulation signal PWM cannot always be maintained at a high-level, and the inductor current IL cannot be continuously outputted. Therefore, it takes longer time to provide enough energy to pull back the output voltage VOUT and causes some problems of under shoot and poor output voltage stability. Although some technologies such as skipping off-time can be used to solve these problems, the circuit complexity and manufacturing costs of the power converter 1 will also increased correspondingly.